A seal mechanism in a linear guide apparatus is a mechanism to provide a seal between a guide rail, and a slider main body and end caps straddling on the guide rail, so as to prevent leakage of an internal lubricant and to prevent entering of external dust. For such purposes, various means have been conventionally employed, and PTL 1 discloses one of such means. Under seals (side surface seals 800 recited in PTL 1) extending in an axial direction are attached to both arm portions of a slider main body and end caps straddling on a guide rail. In this example, the under seals are attached in such a way that each of the under seals is fitted into a retainer, and the retainer is attached to the slider main body and the end caps. Thus, an end surface of the retainer is flush with an end surface of the under seal. Although not illustrated, as recited in PTL 1 at paragraph 0023, a side seal is closely fitted to the end surface of the retainer without gaps, and thus the end surface of the under seal and the side seal are closely fitted to each other. In this manner, a seal is provided between the guide rail, and the slider main body and the end caps straddling on the guide rail.